1. Field of the Invention
The present invention relates to a retrofocus type wide angle lens and more particularly a retrofocus type wide angle lens which is simple in construction yet has a high f number.
2. Related Background Art
As wide angle lenses for single-reflex cameras, various types each of which consists of five negative, positive, nagative, positive and positive groups arranged in the order from an object and which have f number of the order of 2.8 have been devised and demonstrated. Of the wide angle lenses, the wide angle lens whose f number is improved is disclosed in, for instance, U.S. Pat. No. 4,113,358 and the wide angle lens which consists of five groups and five elements or five groups and six elements in order to decrease the number of elements as practically as possible, thereby making the lens light in weight is disclosed in Laid-Open Japanese patent application No. 55-147607.
However, in the case of the wide angle lens disclosed in U.S. Pat. No. 4,113,358, it is made compact in size while attaining an f number of the order of 2.0, but in the case of the lens having such a high f number, the lens elements are increased to seven constituting five groups and furthermore the first group which has the largest diameter consists of cemented elements so that from the standpoint of making the lens light in weight, such arrangement is disadvantageous. The reason is that the focal length of a positive lens cemented in the second group is short. In the case of such arrangement, if the first group consists of one element, it becomes extremely difficult to correct higher orders of spherical aberration, causing flare. Moreover in order to correct higher orders of spherical aberration, the correction of aberration is made on the first group so that the correction of distortion becomes unsatisfactory.
In the case of the wide angle lens disclosed in the above-mentioned Laid-Open Japanese patent application, it can attain the highest f number of the order of 2.0, but the overall length of the lens is relatively longer so that the wide angle lens cannot be said compact in size. The reason is that in order to attain the highest f number of 2.0, the lens system is enlarged so that the correction of spherical aberration and coma aberration may be made satisfactorily.
As described above, in the cases of the conventional wide angle lenses, there exists the problem that in order to make a lens compact in size yet attain the highest f number of the order of 2.0, the lens elements must be increased in number or in order to decrease the number of lens elements while retaining the highest f number of the order of 2.0, the lens system must be enlarged.